@QmlExtended

Declares that the enclosing type uses the type passed as an extension to provide further properties, methods, and enumerations in QML. This takes effect if the type is exposed to QML using a QmlElement() or QmlNamedElement() decorator.

QML_IMPORT_NAME = "com.library.name"
QML_IMPORT_MAJOR_VERSION = 1
QML_IMPORT_MINOR_VERSION = 0 # Optional

class LineEditExtension(QObject):
    pass

@QmlNamedElement("QLineEdit")
@QmlExtended(LineEditExtension)
@QmlForeign(QLineEdit)
class LineEditForeign(QObject):
    ...

Afterwards the class may be used in QML:

import com.library.name 1.0

QLineEdit {
    left_margin: 10
}